Reverse in situ combustion process for the recovery of oil



Aug. 8, 1967 H. w. PARKER 3,334,637

REVERSE IN SITU COMBUSTION PROCESS FOR THE RECOVERY OF OIL Filed Sept.28, 1964 our on. FLO4W IN BURNED OUT AREA I6 id FIG. 3

l8 12 d REVERSE b COMBUSTION INVENTOR. H.W. PARKER United States PatentO" 3,334,687 REVERSE IN SITU COMBUSTION PROCESS FOR THE RECOVERY OF OILHarry W. Parker, Bartlesville, Okla, assignor to Phillips PetroleumCompany, a corporation of Delaware Filed Sept. 28, 1964, Ser. No.399,607 7 Claims. (Cl. 166-2) This invention relates to a method forproducing oil from an oil-bearing subterranean stratum by in situcombustion.

Heavy oil or tar strata are found in various areas and oil recoverytherefrom is especially diflicult. During primary production of suchstrata, the normal rate of production is too slow to render the recoveryof oil economical. The low gravity of the in-place oil and the slowproduction thereof make it desirable to speed up production and/or toupgrade the oil prior to shipment from the field.

The instant invention is concerned with a method or process by whichreverse in situ combustion can be effected prior to depletion of thereservoir pressure or as an initial primary recovery technique.

Accordingly, it is an object of the invention to provide an improvedprocess for recovering oil from a stratum containing low gravity crude.Another object is to provide an improved in situ combustion process forthe recovery of highly viscous crude oil. Other objects of the inventionwill become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises injecting air or othercombustion-supporting gas into the stratum, to be produced thru aproduction well penetrating the stratum and producing one or more offsetinjection wells (preferably under a pressure at least as high as thereservoir pressure) until air appears in the produced oil and reservoirfluids. The injection thru the production Well is then terminated andinjection thru the injection well(s) is initiated and continued untilair is produced in the production well. Thereafter, the stratum isignited adjacent the production well by means well known in the art andinjection thru the injection well(s) is continued so as to feed thecombustion zone around the production well and move same toward theinjection well(s) in a reverse burning process. Fuel gas may beincorporated in the injected air to facilitate ignition at theproduction well. After the combustion zone has been moved thru asubstantial portion of the well pattern by the reverse burning step, airinjection is terminated and the injection well(s) is shut-in and theback pressure on the production well is substantially reduced so as topermit invasion of oil into the well pattern from the area surroundingthe well pattern. There will be some invasion of oil from the stratumimmediately outside of the combustion zone when this combustion zone hasnot reached the injection well(s). The invading oil is heated andcracked as it enters the combustion zone and the hot burned-out areabebind the combustion zone. The heated and upgraded oil is then producedby renewing air injection thru the injection well(s) while maintainingsubstantial back pressure on the production well to prevent furtherinvasion of oil until after the already invaded oil has been produced.Air injection is continued thru the injection well until the injectedair reaches the vicinity of the production well and renews combustion inthe hot stratum which feeds on the unproduced oil resulting from theinvasion. The con tinuance of the injection of air at this stage of theprocess again moves a combustion zone outwardly from the production welltoward the injection well(s) and this combustion zone can be moved tothe injection well(s). At this stage of the process the steps of closingin the injection well(s) and reducing the back pressure on the pro-3,334,687 Patented Aug. 8, 1967 ice duction well again allows oil toinvade the well pattern and the com-bustion zone as well as the hotburned-out area behind the combustion zone and the step of pro ducingthe resulting heated and upgraded oil is repeated.

The foregoing process and repetition thereof is continued until furtherinvasion of oil into the well pattern is so limited that the process isno longer economically feasible. At this time a well pattern in anotherarea of the field is produced in a similar manner. It is also feasibleto simultaneously conduct the in situ combustion process in severalspaced-apart well patterns in the field.

The process described may be effected in any suitable well pattern suchas 3-, 4-, 5-, 6-, 7-, 8-, 9-spot or similar pattern. It may also beeffected between two parallel rows of wells utilized as injection wellswith an intervening roW of production wells. The well spacing in thepattern is preferably closer than would normally be economical forreverse combustion due to the low gravity of the oil being produced.

The termination of air injection thru the injection wells and reductionof back pressure on the production well is maintained until asubstantial portion of the hot straturn has been invaded by the oil andcooled thereby. Normally, when the injected air following the invasionstep breaks thru into the remaining hot stratum near the productionwelhcountercurrent ignition occurs. In the event, there is notsufiicient hot stratum about the production well for ignition when airappears at the production well, reverse ignition can be accomplished inthe normal manner by again igniting the stratum adjacent the productionwell.

Eventually, upon repetition of the process, the reservoir pressurebecomes too low for oil invasion and production into the burned stratumat an economic rate. It has been found in field operations that a 20fold increase in perrneability may occur when reverse burning ispracticed and that considerable expansion of the stratum in excess ofsimple thermal expansion also occurs. This is probably a partialexplanation of the success of repeated reverse burning of the samestratum. In strata in which the reverse combustion is practiced andwhich results in substantial coke deposits, the coke may be burned-outby direct or forward in situ combustion.

It is significant in the instant process that after air permeability hasbeen established between wells in a pattern, continuous production ofreversally cracked oil is maintained in a manner which utilizes theresidual heat of reverse burning. This process also enables theproduction of natural gas in the reservoir without combustion productioncontaminants during the period of oil invasion of a previously .burnedarea of the stratum.

The establishment of air permeability requires a substantial amount oftime compared with such operation in a stratum containing high gravityoil, however, close well spacing is economically feasible since thewells are repeatedly used for the reverse burning operation. Inobtaining air permeabiltiy, it is feasible to first inject water todsplace the viscous oil and then displace the water with air whichrenders the operation more economical. Another economy in compressioncosts is made by using turbines to recover work from the gases producedunder high back pressure at the production well.

A further embodiment of the process comprises maintaining back pressureon the production well somewhat lower than the normal formation pressureso that a portion of the stratum oil outside of the burning zone iscontinually fed into the burning zone from the stratum. This results inincreased rates of oil production and reduced compression costs.

A further embodiment of the process comprises spacing the air injectionwells sufficiently close together and injecting at a sufficiently highpressure so that little or no oil can flow into the pattern from thestratum although the production well pressure is low. Then, when thecombustion front reaches the injection wells and air injection isterminated, the reverse burning continues beyond the injection wells onair stored in the stratum. After this air is exhausted, the oil flowsinto the hot stratum as previously disclosed and reverse burning is thenrepeated.

A more complete understanding of the invention may be had by referenceto the accompanying schematic drawing in which FIGURES lshow the varioussteps of the process as applied to a 5-spot well pattern.

In each of the figures, a central well is surrounded by ring wells 12,14, 16, and 18.

FIGURE 1 illustrates the first step of the process wherein air isinjected through central well 10 and fluids are being produced throughring wells 12, 14, 16, and 18. In this stage of the process, waterinjection may precede air injection to more eflectively displace oilfrom the well pattern and establish communication between the centralwell and the ring wells. Likewise, any other displacing fluid may beutilized preceding the air injection. It is also possible to performstep 1 by injecting through the normal injection wells and producingthrough the normal production well. This procedure requires that all ofthe oil within the ring of wells be produced through the central well,however, which would be a very slow operation if the oil were veryviscous.

FIGURE 2 illustrates the next phase or step of the process after air isproduced from the ring wells by the first step illustrated in FIGURE 1.Substantial backpres sure is maintained on well 10 while injecting airthrough the ring wells until air is produced at well 10 at which timetheoil in the stratum adjacent well 10 is ignited in conventionalmanner. During this air injection phase, a fuel gas such as natural gasmay be incorporated in the injected air to facilitate ignition at well10. Burning a charcoal pack soaked with heavy oil in well 10 whileinjecting air containing fuel through the ring wells is a sure method ofestablishing a combustion zone adjacent well 10 which is identified bynumeral 20.

FIGURE 3 illustrates the next phase or step of the process wherein airinjection through ring wells 12, 14, 16, and 18 is continued after theignition step of FIGURE 2 so as to move a combustion zone 22 radiallyoutward from well 10 toward the ring wells within the stratum. Thecombustion zone 22 may be moved completely to the injection or ringwells 12, 14, 16, and 18 or it may be moved only through a substantialportion of the well pattern before effecting the succeeding step.

FIGURE 4 illustrates the next step in the process which involvesterminating injection of air through wells 12, 14, 16, and 18 andshutting these wells in while reducing the back pressure on productionwell 10 thereby causing the invasion of oil into the combustion zone andthe burnedout area of the stratum behind the combustion zone. Thisinvasion of oil results in heating, cracking, and vaporizing the crackedhydrocarbons which are then produced by the step illustrated in FIGURE5. As is shown in FIG- 5, air is again injected through the ring wellsso as to drive the heated and cracked hydrocarbon material resultingfrom the invasion of oil into the hot area of the pattern intoproduction well 10 from which same is recovered. Continued injection ofair through the ring wells results in renewing the combustion zone 24adjacent production well 10 and further injection of air moves theresulting combusion zone by reverse drive out into the formation as inthe step illustrated in FIGURE 3. The steps of FIGURES 4 and 5 are thenrepeated until the reservoir. pressure is too low for further economicaloperation.

The term substantial back pressure on the production well when used inconnection with the combustion step of this invention does not mean aparticular pressure with respect to the reservoir pressure. It means asuflicient back pressure to prevent large amounts of oil from passingthrough the combustion zone and extinguishing it. To

4 illustrate, if a nine-spot pattern were used, the ring of injectionwells would shield the production well from excess oil invasion and thusthe production well back pressure could be low. In contrast, in usingonly one injection well, the back pressure on the production Well wouldhave to approach that of the reservoir to prevent oil production fromextinguishing the combustion zone.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

I claim:

1. A process for producing fuid hydrocarbons from a subterranean stratumhaving substantial reservoir pressure and containing a viscous crudeoil, said stratum being penetrated by a well pattern including aproduction well and at least one oflset injection well, which comprisesthe steps of:

(a) injecting air into said stratum thru said production well until airis produced through said injection well;

(b) thereafter, terminating injection through said production well andinjecting air through said injection well so as to produce air throughsaid production well while maintaining substantial back pressure on saidproduction Well;

(c) thereafter, igniting said stratum adjacent said production well toform a substantialcombustion zone;

(d) thereafter, continuing air injection through said injection wall asin step (b) and maintaining substantial backpressure on said productionwell so as to feed the combustion zone of step (c) and move same intosaid stratum away from said production well to an area adjacent saidinjection Well;

(e) following step (d), reducing the back pressure on said productionwell to substantially less' than said reservoir pressure so as to allowoil flow into the combustion zone from the surrounding stratum 2. Theprocess of claim 1 wherein substantial back' pressure is again appliedto said production well after invading oil has occupied a substantialportion of the hot area intermediate the injection and production wellsand air injection through said injection well is renewed so as to driveheated and cracked hydrocarbons into said production well; and said airinjection is continued so as to ignite the stratum adjacent saidproduction wall and a combustion zone is again moved toward saidinjection well with production being effected through said productionwell.

3. The process of claim 1 wherein step (a) is preceded by the step ofinjecting water into the production well and producing through theinjection well under substantial back pressure until a substantialportion of the oil in the intervening stratum has been displaced.

4. A process for producing fluid hydrocarbons from a subterraneanstratum containing a viscous crude oil and having substantial reservoirpressure, said stratum being penetrated by a well pattern including acentral production well and a ring of oflset injection wells, whichcomprises the steps of:

(a) injecting air into said well pattern through said central well untilair is produced through said ring wells;

(b) thereafter, terminating air injection through said central well andinjecting air through said ring wells, while maintaining said centralwell under substantial back pressure, until air is produced through saidcentral well;

(c) thereafter, igniting said stratum adjacent said central well andfeeding air to the ignited area through said ring wells so as to movethe resulting combustion zone through said stratum to an area adjacentsaid ring wells to burn out at least a substantial portion of said wellpattern;

((1) thereafter, terminating air injection through said ring wells andreducing the back pressure on said production well substantially belowsaid reservoir pressure so as to permit oil to invade the hot zone insaid well pattern thereby heating and cracking the invading oil; and

(e) thereafter, producing the heated hydrocarbons from step (d).

5. The process of claim 4 wherein step (e) comprises passing air throughsaid pattern from the ring wells to the central well.

6. The process of claim 5 wherein combustion in said stratum issubstantially terminated during step (d) and air injection in step (e)is continued until oil in the stratum is again ignited and a combustionzone is moved through said pattern toward said ring wells.

7. A process for producing fluid hydrocarbons from a subterraneanstratum having substantial reservoir pressure and containing a viscouscrude oil, said stratum being penetrated by a well pattern including aproduction well and at least one offset injection well, which comprisesthe steps of:

(a) injecting air into said stratum through said production well untilair is produced through said injection well while maintainingsubstantial back pressure on said injection well;

'(b) thereafter, terminating injection through said production Well andinjecting air through said injection well so as to produce air throughsaid production well while maintaining substantial back pressure on saidproduction Well;

(c) thereafter, igniting said stratum adjacent said production well toform a substantial combustion zone;

References Cited UNITED STATES PATENTS Pelzer 16611 Crawford et al.16611 Pryor 16611 Reed et al. 166-11 Campion et al 16611 Trantham et al.166-11 X CHARLES E. OCONNELL, Primary Examiner.

ERNEST R. PURSER, STEPHEN J. NOVOSAD,

Examiners.

7. A PROCESS FOR PRODUCING FLUID HYDROCARBONS FROM A SUBTERRANEANSTRATUM HAVING SUBSTANTIAL RESERVOIR PRESSURE AND CONTAINING A VISCOUSCRUDE OIL, SAID STRATUM BEING PENETRATED BY A WELL PATTERN INCLUDING APRODUCTION WELL AND AT LEAST ON EOFFSET INJECTION WELL, WHICH COMPRISESTHE STEPS OF: (A) INJECTING AIR INTO SAID STRATUM THROUGH SAIDPRODUCTION WELL UNTIL AIR IS PRODUCED THROUGH SAID INJECTION WELL WHILEMAINTAINING SUBSTANTIAL BACK PRESSURE ON SAID INJECTION WELL; (B)THEREAFTER, TERMINATING INJECTION THROUGH SAID PRODUCTION WELL ANDINJECTING AIR THROUGH SAID INJECTION WELL SO AS TO PRODUCE AIR THROUGHSAID PRODUCTION WELL WHILE MAINTAINING SUBSTANTIAL BACK PRESSURE ON SAIDPRODUCTION WELL; (C) THEREAFTER, IGNITING SAID STRATUM ADJACENT SAIDPRODUCTION WELL TO FORM A SUBSTANTIAL COMBUSTION ZONE; (D) THEREAFTER,CONTINUING AIR INJECTION THROUGH SAID INJECTION WELL AS IN STEP (B) ANDMAINTAINING A SUBSTANTAIL BACK PRESSURE ON SAID PRODUCTION WELL BUTBELOW THE EXISTING RESERVOIR PRESSURE SURROUNDING SAID WELL PATTERN SOAS TO FEED AND MOVE THE COMBUSTION ZONE OF STEP (C) AWAY FROM SAIDPRODUCTION WELL TO AN AREA ADJACENT SAID INJECTION WELL WHILE PERMITTINGINVASION OF OIL FROM THE SURROUNDING STRATUM INTO THE COMBUSTION ZONE;AND (E) RECOVERING THE HYDROCARBONS PRODUCED BY THE FOREGOING STEPSTHROUGH SAID PRODUCTION WELL.